1. Field of the Invention
The present invention relates to a direct type of tire air pressure detection system which directly attaches transmitters provided with pressure sensor to wheels to which tires are attached, has the transmitters send out detection signals from the pressure sensors, and has a receiver attached to the chassis side receive them so as to detect the tire air pressure.
2. Description of the Related Art
In the past, one type of tire air pressure detection system has been the direct type. In this type of tire air pressure detection system, the wheel side where tires are attached has transmitters provided with pressure sensors directly attached to them. The chassis side has antennas and a receiver provided at it. When detection signals from the pressure sensors are sent from the transmitters, the detection signals are received by the receiver through the antennas and the tire air pressure is detected (for example, see Japanese Unexamined Patent Publication (Kokai) No. 14-257661).
In this type of tire air pressure detection system, transmitters are attached to all of the wheels while antennas are provided near each of the transmitters. Therefore, the antennas receive the detection signals sent from the corresponding transmitters.
However, if the transmission power of a transmitted detection signal is large, as shown by the broken line in the schematic view of the receivable range shown in FIG. 8A, the receivable range of the detection signal sent by one transmitter J1 will extend up to an antenna J2 corresponding to another transmitter J1 and a detection signal sent from the one transmitter J1 will end up being received by the other antenna J2 not corresponding to it.
Further, even if not an antenna J2 of the same vehicle, for example, as shown by the broken line in the schematic view showing crosstalk with a nearby vehicle as shown in FIG. 8B, the detection signal sent by a transmitter J1 attached to the wheel of a vehicle J3 may be received by an antenna J5 corresponding to another vehicle J4. In this case, the other vehicle J4 will end up detecting the air pressure of a tire attached to a wheel of the vehicle J3 different from itself as the pressure of its own wheel J4, i.e., crosstalk will occur.
If crosstalk occurs in this way, there is the problem that the tire air pressure will not be able to be accurately detected.